Document the new canvasKitVariant runtime configuration (#8475)

Add documentation for the new
[`canvasKitVariant`](https://github.com/flutter/engine/blob/0776f38b87137ad2535d77e91a79b8b6c80f16fb/lib/web_ui/lib/src/engine/configuration.dart#L221-L224)
runtime configuration.

Closes flutter/flutter#123048

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---------

Co-authored-by: Anthony Sansone <[email protected]>

src/development/platform-integration/web/initialization.md

@@ -130,6 +130,7 @@ You can pass in the following (optional) parameters:
 | Name | Description | Dart Type |
 |-|-|-|
 |`canvasKitBaseUrl`| The base URL from where `canvaskit.wasm` is downloaded. |`String`|
+|`canvasKitVariant`| The variant of CanvasKit to be downloaded. Available options are:<br><br>1. `auto`: Chooses the most optimal variant for the browser. The option defaults to this value.<br><br>2. `full`: Downloads the full variant of CanvasKit that works in all browsers.<br><br>3. `chromium`: Downloads a smaller variant of CanvasKit that uses APIs compatible with Chromium. **_Warning_**: Don't use the `chromium` option unless you plan on only using Chromium-based browsers. |`String`|
 |`canvasKitForceCpuOnly`| When `true`, forces CPU-only rendering in CanvasKit (the engine won't use WebGL). |`bool`|
 |`canvasKitMaximumSurfaces`| The maximum number of overlay surfaces that the CanvasKit renderer can use. |`double`|
 |`debugShowSemanticNodes`| If `true`, Flutter visibly renders the semantics tree onscreen (for debugging).  |`bool`|